Circulating Melanoma Cell Subpopulations: Their Heterogeneity and Differential Responses to Treatment

Abstract

Metastatic melanoma is a highly heterogeneous tumor; thus, methods to analyze tumor-derived cells circulating in blood should address this diversity. Taking this into account, we analyzed, using multiparametric flow cytometry, the co-expression of the melanoma markers melanoma cell adhesion molecule and melanoma-associated chondroitin sulphate proteoglycan and the tumor-initiating markers ATP-binding cassette sub-family B member 5 (ABCB5), CD271, and receptor activator of NF-κβ (RANK) in individual circulating tumor cells (CTCs) from 40 late-stage (III-IV) and 16 early-stage (I-II) melanoma patients. CTCs were heterogeneous within and between patients, with limited co-expression between the five markers analyzed. Analysis of patient matched blood and metastatic tumors revealed that ABCB5 and RANK subpopulations are more common among CTCs than in the solid tumors, suggesting a preferential selection for these cells in circulation. Pairwise comparison of CTC subpopulations longitudinally before and 6-13 weeks after treatment initiation showed that the percentage of RANK(+) CTCs significantly increased in the patients undergoing targeted therapy (N=16, P<0.01). Moreover, the presence of ⩾5 RANK(+) CTCs in the blood of patients undergoing targeted therapies was prognostic of shorter progression-free survival (hazards ratio 8.73, 95% confidence interval 1.82-41.75, P<0.01). Taken together, our results provide evidence of the heterogeneity among CTC subpopulations in melanoma and the differential response of these subpopulations to targeted therapy.

Figure 1

Marker expression on the surface of melanoma cell lines. (a) Metastatic and primary melanoma cell lines were tested for the expression of ABCB5, CD271, RANK, MCSP, and MCAM by flow cytometry. The percentage (mean±SEM) of positive cells obtained in three independent experiments was determined for each marker. Representative dot plot of flow cytometric staining of (b) MCSP on A2058 and SK-MEL-5 cells or (c) RANK on A2058 and MM540 cells. The corresponding isotype controls are also depicted. ABCB5, ATP-binding cassette sub-family B member 5; MCAM, melanoma cell adhesion molecule; MCSP, melanoma-associated chondroitin sulphate proteoglycan; RANK, receptor activator of NF-κβ.

Figure 2

Analysis of circulating tumor cells (CTCs) in melanoma patients and healthy controls (a) Total number of cells detected in 4 ml of whole blood expressing any of the markers in melanoma patients at late (N=40) and early (N=16) stages of the disease and in healthy controls (N=15). The mean is indicated for each group. An omnibus Kruskal–Wallis test produced a P-value=0.07. Mann–Whitney tests were performed and the P-value indicated for each comparison. (b) Number of cells in 4 ml of blood corresponding to each CTC subpopulations identified. Each bar represents a single melanoma patient. Absent bars represent patients in which CTCs were not detected. ABCB5, ATP-binding cassette sub-family B member 5; MCAM, melanoma cell adhesion molecule; MCSP, melanoma-associated chondroitin sulphate proteoglycan; RANK, receptor activator of NF-κβ.

Figure 3

Receptor activator of NF-κβ (RANK) and ATP-binding cassette sub-family B member 5 (ABCB5) expression in metastases from patients with circulating tumor cells (CTCs) expressing these markers. Immunofluorescence staining of metastatic melanoma tissue corresponding to patients MM15 (a) and MM26 (b). MART-1 staining was used to identify melanoma cells. Original magnification × 400. Scale bar=20 μm.

Figure 4

Prognostic significance of RANK⁺ cells. (a) Total CTC counts and (b) the percentage of CTCs expressing MCAM-MCSP-CD271-ABCB5-RANK+ were quantified in 29 patients with unresectable metastatic melanoma at baseline and 6–13 weeks after treatment initiation. Open circles indicate patients treated with immunotherapy (N=13); ipilimumab (N=10), pembrolizumab (N=2), and nivolumab (N=1). Black circles indicate patients treated with targeted therapies (N=16); vemurafenib (N=4), dabrafenib/trametinib (N=10), and dabrafenib (N=2). The P-values of Wilcoxon matched-pairs signed rank tests are indicated for each comparison. (c and d) Kaplan–Meier curves for progression-free survival of metastatic melanoma patients undergoing targeted therapies (N=16). Patients were stratified as those with ⩾5 and <5 RANK⁺ cells in 4 ml of blood at (c) baseline (before therapy) and (d) after therapy initiation. The long rank P-values are indicated. ABCB5, ATP-binding cassette sub-family B member 5; CTC, circulating tumor cell; MCAM, melanoma cell adhesion molecule; MCSP, melanoma-associated chondroitin sulphate proteoglycan; RANK, receptor activator of NF-κβ.

Figure 5

RANKL stimulation effect in vemurafenib sensitivity. (a) Sensitivity to vemurafenib/PLX4032 of A2058 and 1205Lu melanoma cells pre-incubated with or without RANKL at 2.5 μg ml⁻¹ for 12 hours prior to treatment. Cells were treated with approximately two times their respective 50% inhibitory concentration (IC₅₀), 60 μM for A2058, and 1 μM for 1205Lu. Percentage of inhibition was calculated relative to cells not exposed to vemurafenib. (b) Dose-response curve of vemurafenib growth inhibition of cells pre-incubated with RANKL at 2.5 and 0.5 μg ml⁻¹. Experiments were performed in triplicate and the mean and SD are indicated for each set. RANK, receptor activator of NF-κβ.